Hi,
I've been a Cakewalk user since 1.0, but over the years I migrated away from doing much MIDI-based work, instead using SONAR to mix the band's audio tracks. Recently, I'm doing some solo projects, and importing MIDI generated elsewhere (mainly drums and bass, a little piano and strings), then recording guitars, vocals, keyboards, etc. 
 
The point is, I confess that I have no sophistication whatsoever when it comes to effectively using softsynth(s) to derive the MIDI tracks. In fact, I can really never fathom the myriad of parameters, which are frankly overwhelming, and most of the time can get no sound out of them at all. Consequently, I have resigned myself to using Cakewalk TTS, just because I can get, you know, "sound" out of it. But I'm sure I'm missing a wealth of potential simply due to my ignorance. 
 
I googled the internets, but only seemed to turn up SONAR 3 documentation which was bare bones "how to add a soft synth". I'm really looking for guidance/literature/tutorials that describe the plethora of synths available, how to use them most effectively, etc.
 
I have Sonar Platinum, with a ton of plugins, but most are heavily UNDER-utilized.
 
Any pointing me to the right resources, learning tools, etc. would be greatly appreciated.

You won't regret buying this...
 
http://www.amazon.com/SONAR-X3-Power-Comprehensive-Guide/dp/1305090195/ref=sr_1_1?ie=UTF8&qid=1440679189&sr=8-1&keywords=sonar+power
 
Regards,
Dan

TTS is a general midi (GM) synth, easy to use but which sounds, are, less than other, harder to use synths.  I don't know what "midi" you are getting from others, but channel 10 midi is usually reserved for drums, and Addictive Drums (which comes w/ SP) is a very nice "drum machine." That should be your first stop - you'll find your productions sound more acoustic using it.
 
DimPro is Cakewalk's sample playback synth, as good as most others for acoustic sounds.  Other synths have larger libraries and more and extensive additional libraries, but as a bog-standard acoustic emulation engine it is the best you'll find in SP.  And there are more libraries available on the Cake site to round out your sounds or if you need something special.  However, there is probably not a 5 gig library of triangles if you are doing orchestral work.  Get Kontakt and a bunch of libraries if you are going down that road.
 
Rapture is Cake's synth.  It too is a sample playback synth, but the controls, tho similar to DimPro, are more extensive and w/ lots 'o step sequencers to make it easy to do loop-based or dance type music.  The sample selection is also biased toward synths and keyboards rather than acoustic emulations (in fact, the "missing" organs one would think one would find in DimPro are in Rapture).  And both use SFZ format and both sound "bright," to my ears.  They cut through a mix, but sometimes you don't want that - like too much transistor distortion - if everything is bright and sharp nothing stands out in a song. 
 
Z3TA is Cake's analog (synth) emulation and is a bit cutting, too, but sounds different than the SFZ synths and is more modular.  I prefer Alchemy for deeper, smoother analog synth sounds, but you'll have to use Logic and Apple to get that one now.  There is also the newer Z3TA 2+ and Rapture Pro (a great combo of Dimpro and Rapt), and older analog emulations that others like.  I'm not a big fan of them, but they are certainly usable.  So you should find decent presets among the several thousand available between all the SP synths.  
 
Figuring out which you like is the key - they are there.  But no one can tell you which synth and preset to use unless they are right there with you when making the choice, but the above should provide some guidelines where to start listening.

Thanks. These were extremely helpful replies, and now I'm itching to get in (after downloading Huntington, or whatever town this latest upgrade is named after ) and use these suggestions. 
As for SONAR X3 Power!, well, turns out its on my Kindle, and has been. For enough time that I had forgotten I owned it. Like Gandalf said about Barliman Butterbur: "His memory is like a lumber-room: thing always wanted buried". This is easily true of me as well. 

A book I've found helpful is Cakewalk Synthesizers by Simon Cann. It's older, but it walks you through many synths still part of SPlat and does a good job of covering each of the parameters (variables) of each synth. The first synth covered, Triangle II, can be found for download, and I believe all but one are still included. It also covers soundfonts. If you get this, and can't find the Triangle II synth download, PM me, and I'll put it in my cloud for you to grab.
 
http://www.amazon.com/Cak...s-Power/dp/1435455649/

found it. got it. thanks!